Crosslinking agents comprising s-triazine compounds are known in the art. Koral et al., U.S. Pat. No. 3,661,819, for example, disclose a family of s-triazine curing agents comprising fully or partially alkylated melamine-formaldehyde compounds having the formula: ##STR1## or (ii) a benzoguanamine compound of the formula: ##STR2## wherein R is hydrogen or alkyl of from 1 to 12 carbon atoms. It is also known to use oligomers of such compounds, which are low molecular weight condensation products containing for example two, three or four triazine rings, joined by --CH.sub.2 OCH.sub.2 -- linkages, as well as mixtures of any of the foregoing. These are used to self-condense or used to cure active hydrogen-containing materials, especially polymers which contain carboxyl groups, alcoholic hydroxy groups, amide groups and groups convertible to such groups, such as methylol groups. Coatings containing melamine-formaldehyde crosslinkers have good hardness and high crosslink density. The coatings generally do not discolor upon exposure to light, especially ultraviolet from sunlight or other sources, moisture or oxygen. A serious shortcoming of these crosslinkers is that they tend to liberate formaldehyde on curing which is objectionable to both formulators and end-users. Moreover, coatings crosslinked with these materials have a tendency to brittleness, at least as compared with other coatings such as polyurethane coatings.
Crosslinking agents based on beta-hydroxyalkyl carbamates are known from Valko, U.S. Pat. No. 4,435,559. Valko describes curable compositions comprising a bis(beta-hydroxyalkyl carbamate) crosslinker, an active-hydrogen material and a cure catalyst. The Valko crosslinkers are prepared from diisocyanate intermediate. The coatings derived from aromatic blocked diisocyanates are not light stable in outdoor use. Although coatings prepared therefrom are more flexible than the aforementioned melamineformaldehyde based coatings, they suffer from poor crosslink density, poor hardness and poor organic solvent resistance. Moreover, they require use and handling of hazardous and toxic isocyanate materials.
Another patent dealing with beta-hydroxyalkyl carbamate crosslinkers is Jacobs, III, Parekh and Blank, U.S. Pat. No. 4,484,994, which discloses their use in cathodically electrodepositable coating compositions.
Accordingly, to overcome certain drawbacks of the prior art crosslinkers, it is an object of the present invention to provide new and improved crosslinking agents for use with active hydrogen containing materials and polymers which impart the hardness, toughness, solvent resistance and light stability of melamine-formaldehyde crosslinkers but without the brittleness, and which possess the abrasion resistance and flexibility of polyurethane coatings.
It is another object of the present invention to provide melamine-urethane crosslinkers for curable systems which are formaldehyde and isocyanate free.
It is a further object of the present invention to provide curable coating compositions for use in powder coating, electrocoating and solvent-borne coating applications.